Mikasa's combat capabilities don't change the fact that her character is completely defined by a man. She's a non-sucky Sakura.
Say what you will about female One Piece characters, but none of them are completely defined by a man.
Okay, first off, this is false.
Mikasa is defined by something other than Eren. She's also defined is her duties as a Survey Corps soldier. I'm not even extrapolating on anything, because this is a plot point that comes up multiple times. The group wants to do a mission of some kind, and and Mikasa has Eren as a priority, and they need to remind her that she has other obligations. And she does adhere to those obligations. It's where the internal struggles of her character lie, when she is torn between those two devotions. It's a major part of her character.
And the problem with Naruto's women wasn't that Sakura was defined by her relationship with a man, but that nearly all the women in that series were. There is nothing wrong with a character whose main core is the love they have for another character. Plenty of male characters, stretching back to ancient myths. The reason that it isn't a men's issue is that we don't generalize that characteristic to ALL men. We understand that men have various interests and devotions. So even if Rory from Doctor Who, or Edward from Twilight, or Snape from Harry Potter, or hundreds of other male cahracters can be said to be defined by their love for a woman, we know that male characters aren't as a rule.
The problem with sexism is that exactly what we do with women, and this is why we have these false notions that a woman can't be attached to a man or otherwise she's just fitting into the mold, ignoring the context of the story she is in. And the reason that Mikasa is a good feminist character, despite her being devoted to a man, that attribute is 1. Not put on any kind of pedestal and the more important 2. While Mikasa may be devoted to a man, this
isn't a statement on women.
So if you want to compare Attack on Titan to Naruto, you can't just cherry pick Mikasa and Sakura, you have to take all female characters of both series and count how many are defined by their relationship with men in each. Out of the main characters, on Naruto's side, we have Sakura, Hinata, Ino, Tsunade. More if you start counting lesser characters. On the Attack on Titan, we have Mikasa. That's it. And Petra, if you want to stretch the definition of main character to the brink of it's breaking point.
Hange is primarily devoted to SCIENCE! and the dicey moral ethics resulting from that. Krista has major identity issues with her family that become major plot points later on. Annie who is devoted to her mission, and has mysterious motivations. Sasha is a street rat that has hunger issues that is devoloping into a warrior. Ymir has a lot more going on for her than love interest, but even if we ignore the other stuff, she's defined by her love towards another
woman. So there's nothing wrong with Mikasa being devoted to Eren as she is, because Attack on Titan doesn't make her devotion isn't a 'woman' thing, it's a 'Mikasa' thing. She is literally the only character of her kind in this regard in the entire series.
To say nothing of the fact that the consequences of a singleminded devotion are treated with a lot of nuance. This obsession that Mikasa is played in a number of ways. In some cases, it can be touching, because such a single minded passion is highly emotional, and a lot of people would like to have anyone care as much about them as Mikasa does about Eren. But at the same time, it's played comedically, with how Mikasa gets jealous and expresses it through violence (like the training ground with Annie). But then it's also sad and pathetic. When she thought Eren died, she lost the will to live. What kind of person is that, to have that little self agency and worth? But then it's also just flat out annoying. Levi and Jean both have to remind her that the fight they're fighting is bigger than just them and she can't just put Eren in front of the world. Seriously, compare the various degrees to which Mikasa's devotion is examined, and tell me how it compares to Naruto where it's played 99% for wangst, and (to my knowledge) how it's never really treated as the fucked up thing it is when Sasuke is so horrifically abusive.
Because Mikasa's devotion is treated with the complexity that befits it, it's good in some cases, bad in a lot of others. It is to say, this features isn't idealized or demonized. It just exists as part of her character, which defines her, yes, but not in a shallow way like in Naruto. It's a complex aspect of her identity that might be the core of who she is, but she's also someone whose growing beyond it, and has faced dilemma's concerning her devotion before. She understands that while Eren might be really important to her, he's not the only important thing to her, and she's not the only person with important things in the world. This makes Mikasa
complex and believable as a fully developed human being. She's well written, in other words.
And
that's what makes her a good feminist character.
And to make this post somewhat relevant to the topic, OP's women aren't often defined by men, I'll give them that. In fact, as far as Nami and Robin go, they're more defined by their mothers than anything. So I approve, but that doesn't mean they don't have a ton of other sexist issues in the series.